Polyoxymethylene resin composition

ABSTRACT

A polyoxymethylene resin composition comprising: 
     (A) 100 parts by weight of a polyoxymethylene copolymer; 
     (B) 0.01 to 7 parts by weight of an amine-substituted triazine compound; and 
     (C) 0.01 to 5 parts by weight of (C-1) polyethylene glycol having an average molecular weight of 10,000 or more and/or (C-2) modified polyolefin wax having an acidic group with an acid value of 0.5 to 60 mg-KOH/g. 
     According to the present invention, there can be provided a polyoxymethylene resin molded product which has low shrink anisotropy, excellent thermal stability and dimensional stability.

DETAILED DESCRIPTION OF THE INVENTION

1. Field of the Invention

The present invention relates to a polyoxymethylene resin compositionfor obtaining a molded product having extremely low shrink anisotropywhen it is left for long time after molding or in a high-temperatureatmosphere and excellent thermal stability.

2. Prior Art

A polyoxymethylene resin is used in a wide variety of application fieldssuch as mechanical, electric and electronic, automobile, constructionmaterial and household goods fields as a typical engineering plastic dueto its excellent mechanical properties, sliding properties, chemicalresistance, fatigue resistance and the like.

It is known that the polyoxymethylene resin has poor thermal stabilitydue to its molecular structure and readily decomposes due to the breakof a main chain caused by depolymerization from the terminal of thepolymer or a thermal oxidization decomposition reaction. Further, sinceformic acid formed by the oxidation reaction of formaldehyde produced bythe decomposition promotes the thermal oxidation decomposition reactionof the polyoxymethylene resin, the thermal stability of thepolyoxymethylene resin is greatly impaired with the result that thepractical applicability of the resin is lost. Therefore, the addition ofan amine-substituted triazine compound typified by melamine, so-called“formaldehyde scavenger”, is essential to the improvement of the thermalstability of the polyoxymethylene resin.

However, the addition of the amine-substituted triazine compound whichis an essential ingredient for the improvement of the thermal stabilityof the polyoxymethylene resin increases the shrink anisotropy of amolded product, and a molded product of the polyoxymethylene resin has alarge molding shrinkage factor because the polyoxymethylene resin hashigh crystallinity. Therefore, the application of the polyoxymethyleneresin in precision parts which require high dimensional stability islimited in most cases and improvement on the shrink anisotropy of theresin has been desired.

As one of the methods for improving the shrink anisotropy of a moldedproduct of this polyoxymethylene resin, attempts are being made to addan inorganic filler such as talc. This method can reduce shrinkanisotropy to a certain degree but it involves such a problem that thecharacteristic properties of the polyoxymethylene resin are impaired,that is, physical properties such as impact resistance deteriorate.

Meanwhile, a technology for blending various resins to improve shrinkanisotropy has been proposed as another method for improving shrinkanisotropy. For example, JP-A 4-108848 (the term “JP-A” as used hereinmeans an “unexamined published Japanese patent application”) proposespolyoxymethylene homopolymer and copolymer compositions comprisingdifferent polyoxymethylenes, which involves such molding problems as areduction in thermal stability and the difficulty of uniformplasticization. JP-A 64-38463 proposes a composition comprising aspecific high-viscosity polystyrene resin, JP-A 4-214756 proposes acomposition comprising a polystyrene-based resin and acryl-based resin,JP-A 6-248163 proposes a composition comprising a polycarbonate-basedresin, a phenol-based polymer compound and a filler, JP-A 6-299046proposes a composition comprising a styrene-based resin, a phenol-basedpolymer compound and a filler, and JP-A 7-292187 proposes a compositioncomprising a polystyrene-based resin having a hydroxyl group, acopolymer of a polyacrylic acid ester and styrene, and a polyfunctionalisocyanate. However, all of these compositions have such a defect thatthe characteristic features of the polyoxymethylene resin are greatlyimpaired as exemplified by reductions in physical properties,deterioration in the surface state of a molded product, a reduction inthermal stability and the like caused by the occurrence of adelamination or layer separation phenomenon, a rise in viscosity andpoor dispersibility.

In view of the above situation, it is an object of the present inventionto provide a polyoxymethylene resin composition which can give a moldedproduct having extremely low shrink anisotropy when it is left for along time after molding or in a high-temperature atmosphere andexcellent thermal stability without impairing the characteristicproperties of a polyoxymethylene resin. It is another object of thepresent invention to provide a polyoxymethylene resin composition whichcan give a molded product required to have high dimensional stability,such as a precision part.

JP-B 37-8816 (the term “JP-B” as used herein means an “examined Japanesepatent publication”) discloses a method for improving the flowability ofa polyoxymethylene resin at the time of molding by adding polyethyleneglycol and JP-A 56-163144 discloses a method for improving the hot waterresistance of a polyoxymethylene resin by adding polyethylene glycol.Surprisingly, the inventors of the present invention have found that amolded product having extremely low shrink anisotropy when it is leftfor a long time or in a high-temperature atmosphere is obtained byselecting polyethylene glycol having a molecular weight larger than aspecific value and adding it to a polyoxymethylene copolymer togetherwith an amine-substituted triazine compound. The present invention hasbeen accomplished based on this finding.

Meanwhile, JP-A 59-51937 and JP-A 60-86155 disclose a method forimproving the dispersibility of carbon black by adding polyolefin wax toa polyoxymethylene resin. Further, JP-A 3-70764 and JP-A 4-224856disclose a method for improving the abrasion resistance of apolyoxymethylene resin by adding polyolefin wax. Further, JP-A 8-3236discloses a method for improving the self-lubrication of apolyoxymethylene resin by adding polyolefin wax. Surprisingly, thepresent inventors have found that a molded product having extremely lowshrink anisotropy when it is left for a long time or in ahigh-temperature atmosphere is obtained by selecting polyolefin waxhaving an acid value higher than a specific value and adding it to apolyoxymethylene copolymer together with an amine-substituted triazinecompound. The present invention has been accomplished based on thisfinding.

That is, the present invention is a polyoxymethylene resin compositionwhich substantially comprises (A) 100 parts by weight of apolyoxymethylene copolymer, (B) 0.01 to 7 parts by weight of anamine-substituted triazine compound, and (C) 0.01 to 5 parts by weightof (C-1) polyethylene glycol having an average molecular weight of10,000 or more and/or (C-2) modified polyolefin wax having an acidicgroup with an acid value of 0.5 to 60 mg-KOH/g.

The present invention will be described in detail hereinunder.

The polyoxymethylene copolymer (A) used in the present invention isgenerally a copolymer containing 0.4 to 40 mol %, preferably 0.4 to 10mol % of oxyalkylene units in the main chain of oxymethylene. Thecopolymer is obtained by polymerizing formaldehyde and/or a cyclicoligomer thereof (for example, trioxan or tetraoxan) as a main monomer/sand a cyclic ether as a copolymerizable component in the presence of apolymerization catalyst.

The cyclic ether used as a copolymerizable component is preferably acompound represented by the following general formula (1).

wherein R₁, R₂, R₃ and R₄ are the same or different and each a hydrogenatom or alkyl group having 1 to 5 carbon atoms, and R₅ is a methylenegroup, oxymethylene group, or methylene group or oxymethylene groupsubstituted by an alkyl group (n is an integer of 0 to 3), or a divalentgroup represented by the following general formula (2) or (3) (n is 1,and m is an integer of 1 to 4).

—(CH₂)_(m)—O—CH₂—  (2)

—(O—CH₂—CH₂)_(m)—O—CH₂—  (3)

Specific examples of the cyclic ether include ethylene oxide, propyleneoxide, 1,3-dioxolan, 1,3-dioxepan, 1,3,5-trioxepan, 1,3,6-trioxocan. Outof these, 1,3-dioxolan is particularly preferred from the view point ofthe thermal stability of the obtained resin composition.

The polymerization catalyst Is a general cationic polymerizationcatalyst. A compound containing boron trifluoride is preferred, asexemplified by a hydrate and coordination complex compound of borontrifluoride. Diethyl etherate of boron trifluoride which is acoordination complex with an ether is particularly preferred.

The polymerization of a polyoxymethylene copolymer can be carried out bythe same device and the same method as those for the copolymerization ofconventionally known trioxan. That is, it can be carried out in a batchor continuous system and can be applied to block polymerization or meltpolymerization which is carried out in the presence of an organicsolvent such as cyclohexane. A reaction tank equipped with a stirrer canbe used for a batch system whereas a kneader, extruder, self-cleaningtype continuous mixer and the like having such a function as quickhardening at the time of polymerization, high stirring ability to copewith heat generation, fine temperature control or self-cleaning toprevent the adhesion of scales are suitably used for continuous blockpolymerization.

The catalyst can be deactivated or removed from the polyoxymethylenecopolymer obtained by a polymerization reaction in accordance with knownmethods which use one selected from primary, secondary and tertiaryamines such as diethylamine, triethylamine, di-iso-propylamine,tri-iso-propylamine, mono-n-butylamine, dibutylamine, tributylamine,piperidine and morpholine, hydroxides of alkali metals and alkali earthmetals, and trivalent organic phosphorus compounds, or an aqueoussolution or an organic solution thereof. Illustrative examples of theorganic solvent include alcohols such as methanol and ethanol; ketonessuch as acetone and methyl ethyl ketone; aromatic compounds such asbenzene, toluene and xylene; and saturated hydrocarbons such ascyclohexane, n-hexane and n-heptane. Out of these, a method fordeactivating a catalyst with a tertiary amine or a trivalent organicphosphorus compound (JP-B 55-42085) are preferred.

It is known that the polyoxymethylene copolymer has poor thermalstability due to its molecular structure and readily decomposes due tothe break of a main chain caused by depolymerization from the terminalof the polymer or a thermal oxidization decomposition reaction. Further,since formic acid formed by the oxidation reaction of formaldehydeproduced by the decomposition promotes the thermal oxidationdecomposition reaction of the polyoxymethylene copolymer, the thermalstability of the polyoxymethylene copolymer is greatly impaired with theresult that the practical applicability of the copolymer is lost.Therefore, the addition of a formaldehyde capturing agent is essentialto the improvement of the thermal stability of the polyoxymethylenecopolymer. Consequently, an amine-substituted triazine compound (B)which is a formaldehyde capturing agent is added to the polyoxymethylenecopolymer in the composition of the present invention.

The amine-substituted triazine compound as the component (B) is at leastone selected from amine-substituted triazines having a structurerepresented by the following general formula (4) and initialpolycondensates of the amine-substituted triazines and formaldehyde.

wherein R₈, R₉ and R₁₀ are the same or different and each a hydrogenatom, halogen atom, hydroxyl group, alkyl group, alkoxy group, arylgroup, hydrogenated aryl group, amino group or substituted amino group,with the proviso that at least one of them is an amino group orsubstituted amino group.

Illustrative examples of the amino-substituted triazine compound or theinitial polycondensate of the amino-substituted triazine compound andformaldehyde include guanamine, melamine, N-butylmelamine,N-phenylmelamine, N,N-diphenylmelamine, N,N-diallylmelamine,N,N′,N″-triphenylmelamine, N,N′,N″-trimethylolmelamine, benzoguanamine,2,4-diamino-6-methyl-sym-triazine, 2,4-diamino-6-butyl-sym-triazine,2,4-diamino-6-benzyloxy-sym-triazine, 2,4-diamino-6-butoxy-sym-triazine,2,4-diamino-6-cyclohexyl-sym-triazine,2,4-diamino-6-chloro-sym-triazine, 2,4-diamino-6-mercapto-sym-triazine,2-oxy-4,6-diamino-sym-triazine(ammeline), N,N,N′,N′-tetracyanoethylbenzoguanamine and initial polycondensates of these and formaldehyde.Out of these, melamine, methylolmelamine, benzoguanamine andwater-soluble melamine-formaldehyde resin are particularly preferred.

The amount of the amine-substituted triazine compound (B) is 0.01 to 7parts by weight, preferably 0.01 to 1 part by weight based on 100 partsby weight of the polyoxymethylene copolymer. When the amount is smallerthan 0.01 part by weight, a stabilizing effect is insufficient and whenthe amount is larger than 7 parts by weight, the obtained molded productdeteriorates in physical properties and has a bad appearance.

The component (C) used in the resin composition of the present inventionis polyethylene glycol (C-1) having an average molecular weight of10,000 or more or modified polyolefin wax (C-2) having an acidic groupwith an acid value of 0.5 to 60 mg-KOH/g. These components (C-1) and(C-2) may be used alone or in combination.

The polyethylene glycol as the component (C-1) is obtained by thering-opening polymerization of ethylene oxide and may have a molecularweight of 10,000 or more, generally 10,000 to 30,000, preferably 10,000to 25,000 when it has a hydroxyl group at a terminal. The polyethyleneglycol used in the present invention may be straight-chain orbranched-chain. When the average molecular weight of the polyethyleneglycol is smaller than 10,000, the effect of reducing shrink anisotropyis hardly observed.

The modified polyolefin wax having an acidic group with an acid value of0.5 to 60 mg-KOH/g (component (C-2)) preferably has an average molecularweight of 30,000 or less. The component (C-2) is obtained by oxidationmodifying or acid modifying polyolefin wax. To produce the modifiedpolyolefin wax, an acidic group is introduced by the oxidation reactionof polyolefin wax, oxidation decomposing polyolefin, introducing a polargroup such as a carboxyl group or sulfonic acid group by reactingpolyolefin wax with an inorganic acid, organic acid or unsaturatedcarboxylic acid, or introducing a monomer having an acidic group duringthe polymerization of polyolefin wax. These are available on the marketunder the name of oxidation modified or acid modified polyolefin wax andcan be easily acquired. The modified polyolefin wax having an acidicgroup preferably has an acid value of preferably 1.0 to 50 mg-KOH/g anda number average molecular weight of generally 500 to 30,000, preferably1,000 to 20,000.

The amount of the component C in the present invention is generally 0.01to 5 parts by weight, preferably 0.01 to 1 part by weight, morepreferably 0.05 to 0.5 part by weight based on 100 parts by weight ofthe polyoxymethylene copolymer. When the amount is smaller than 0.01part by weight, the effect of reducing shrink anisotropy becomesinsufficient and when the amount is larger than 5 parts by weight,strength lowers disadvantageously. The above components C may be usedalone or in combination of two or more.

To further improve the thermal stability of the resin composition of thepresent invention, a steric hindrance phenol (D) is preferably added.The steric hindrance phenol (D) is a compound having at least onestructure represented by the following general formula (5) in themolecule (R₁₁ and R₁₂ are the same or different and each an alkyl groupor substituted alkyl group).

Illustrative examples of the steric hindrance phenol include2,2′-methylene-bis(4-methyl-6-t-butylphenol),4,4′-methylene-bis(2,6-di-t-butylphenol),1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,3,5-di-t-butyl-4-hydroxybenzyl dimethylamine,stearyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonate,diethyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonate,2,6,7-trioxa-1-phosphor-bicyclo[2,2,2]-octo-4-yl-methyl-3,5-di-t-butyl-4-hydroxyhydrocinnamate,3,5-di-t-butyl-4-hydroxyphenyl-3,5-distearyl-thiotriazylamine,2(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole,2,6-di-t-butyl-4-hydroxymethylphenol,2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylallylino)-1,3,5-triazine,N,N′-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamamide),octadecyl-3-(3,5-di-butyl-4-hydroxyphenyl)propionate,1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],pentaerythrithyl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],triethylene glycol-bis[3-(3,5-dimethyl-4-hydroxyphenyl)propionate],triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],triethylene glycol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],2,2′-thiodiethyl-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] andthe like.

Out of these, a compound having at least one structure represented bythe following general formula (6) in the molecule is preferred.

That is, preferred examples of the compound having the structure of theabove general formula (6) includeN,N′-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamamide),octadecyl-3-(3,5-di-butyl-4-hydroxyphenyl)propionate,1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],pentaerythrithyl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],triethylene glycol-bis[3-(3,5-dimethyl-4-hydroxyphenyl)propionate],triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],triethylene glycol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],2,2′-thiodiethyl-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] andthe like.

Out of these, more preferred are1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],pentaerythrithyl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],triethylene glycol-bis[3-(3,5-dimethyl-4-hydroxyphenyl)propionate],triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],triethylene glycol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] and2,2′-thiodiethyl-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate].

The amount of the steric hindrance phenol (D) is generally 0.01 to 5parts by weight, preferably 0.01 to 1 part by weight based on 100 partsby weight of the polyoxymethylene copolymer (A). When the amount issmaller than 0.01 part by weight, the improvement of a stabilizingeffect obtained by adding the component (D) becomes insufficient andwhen the amount is larger than 5 parts by weight, gas is generatedduring molding, or the obtained molded product has a bad appearance.

To further improve the thermal stability of the resin composition of thepresent invention, at least one metal compound selected from the groupconsisting of hydroxides, inorganic acid salts, organic acid salts andalkoxides of alkali metals and alkali earth metals is preferably added.The inorganic acid salts include carbonates, phosphates, silicates andborates. The organic acid salts include lauric acid salts, stearic acidsalts, oleic acid salts or behenic acid salts. The alkoxides arealkoxides having 1 to 5 carbon atoms such as methoxides and ethoxides.Out of these, hydroxides, inorganic acid salts, organic acid salts andalkoxides of alkali earth metals are preferred, and calcium hydroxide,magnesium hydroxide, calcium carbonate and magnesium carbonate are morepreferred.

The amount of at least one metal-containing compound (E) selected fromthe group consisting of hydroxides, inorganic acid salts, organic acidsalts and alkoxides of alkali metals and alkali earth metals isgenerally 0.001 to 5 parts by weight, preferably 0.001 to 3 parts byweight based on 100 parts by weight of the polyoxymethylene copolymer(A). When the amount is smaller than 0.001 part by weight, theimprovement of a stabilizing effect which is obtained by adding thecomponent (E) becomes insufficient and when the amount is larger than 5parts by weight, the obtained molded product deteriorates in physicalproperties and has a bad appearance.

The resin composition of the present invention may contain suchadditives as other stabilizer, nucleating agent, release agent, filler,pigment, lubricant, plasticizer, ultraviolet absorber, flame retardantand flame retarding aid, other resin and elastomer as required insuitable amounts. Illustrative examples of the filler include mineralfillers such as glass beads, wollastonite, mica, talc, boron nitride,calcium carbonate, kaolin, silicon dioxide, clay, asbestos, diatomaceousearth, graphite and molybdenum disulfide; inorganic fibers such as glassfibers, milled glass fibers, carbon fibers, potassium titanate fibersand boron fibers; and organic fibers such as aramid fibers.

Various methods for producing the polyoxymethylene copolymer resincomposition of the present invention may be employed. It is essential tomix or melt knead predetermined components. After predeterminedcomponents are added to a polyoxymethylene copolymer which has beensubjected to the deactivation of a catalyst, they are preferablypre-mixed by a mixer such as a blender or Henschel mixer. Pre-mixing canbe carried out in a dry state or in the form of a moistened product,emulsion or suspension. The moistened product, emulsion and suspensionare prepared by adding water, methanol, acetone, benzene, toluene,cyclohexane or other known solvent to the polyoxymethylene copolymer.

The above components may be added to the polyoxymethylene copolymer (A)in a molten state directly from an addition port formed in the barrel ofan extruder without being pre-mixed.

The predetermined components and the polyoxymethylene copolymer may bemelt kneaded with a kneading machine selected from single-screw andtwin-screw extruders, kneader and Banbury mixer, out of which anextruder is preferred. More preferred is a single-screw or twin-screwextruder which can remove decomposed formalin and impurities at apredetermined reduced pressure from a vent port. Much more preferred isa twin-screw extruder which can remove gas at a predetermined reducedpressure from two or more vent ports.

There are a variety of twin-screw extruders, such as an extruder whoseeach of the two screws turn in the co-rotation or counter-roration, anextruder having a deep screw groove, an extruder having a shallow screwgroove, an extruder having a normal screw, an extruder having a reversescrew and an extruder incorporating a kneading block for improving akneading effect, an extruder having a seal ring for improving adegassing effect and the like. Basically, any extruder is acceptable ifit has melt kneading capability to disperse a stabilizer well and doesnot impair the thermal stability of the polyoxymethylene copolymer. Themelt kneading temperature is generally 160 to 270° C.

EXAMPLES

Reference examples, examples and comparative examples are given todetail the embodiments and effects of the present invention. It shouldbe understood that the present invention is not limited to theseexamples.

Properties of resin compositions shown in examples and comparativeexamples were measured in accordance with the following methods.

(1) Thermal Stability Test

The residence time (minutes) required until silver streaks are formed ata cylinder temperature of 240° C., a mold temperature of 70° C. and amolding cycle of 30 seconds is measured as means of evaluating thesilver streak formation time based on residence in a cylinder using aninjection molding machine having a clamping pressure of 75 tons. Thelarger the value the higher the thermal stability becomes.

“Non-colored” in the table shows the result obtained when a resincomposition is molded without being mixed with carbon black and “blackcolored” shows the result obtained when a resin composition is mixedwith carbon black.

(2) Shrinkage Factor and Anisotropy Test

After a square molded plate with a fan gate measuring 100 mm inlength×100 mm in width×4 mm in thickness is injection molded at acylinder temperature of 200° C. and a mold temperature of 70° C. usingan injection molding machine having a clamping pressure of 75 tons, an Lgate is cut out and left at 23° C. and 50% RH for 48 hours. And then theshrinkage factor (%) and anisotropy (%) of the L gate are obtained fromthe following expressions as the size (mm) of the L gate in a flowdirection represented by D_(P1) and the size (mm) of the L gate in adirection perpendicular to the flow direction represented by D_(V1).

shrinkage factor (flow direction) S _(P)=(D _(P0) −D _(P1))/D_(P0)×100(%)

shrinkage factor (perpendicular direction) S _(V)=(D _(V0) −D _(V1))/D_(V0)×100(%)

anisotropy=S _(P) −S _(V)(%)

D_(P0): size of mold in flow direction (mm)

D_(V0): size of mold in perpendicular direction (mm)

D_(P1): size of molded product in flow direction (mm)

D_(V1): size of molded product in perpendicular direction (mm)

S_(P): shrinkage factor of molded product in flow direction (%)

S_(V): shrinkage factor of molded product in perpendicular direction (%)

Reference Example 1

100 parts by weight of trioxan and 4.5 parts by weight of 1,3-dioxolanas a comonomer were polymerized in a twin-screw kneader having paddleswhich engage each other using boron trifluoride etherate as a catalystand methylal as a chain transfer agent. After the end of polymerization,the catalyst was deactivated with a benzene solution containing a smallamount of triphenyl phosphine and milled to give a polyoxymethylenecopolymer. The polyoxymethylene copolymer had an intrinsic viscosity inp-chloroform (containing α-pinene) at 60° C. of 1.45 dl/g.

Reference Example 2

100 parts by weight of trioxan and 2.5 parts by weight of ethylene oxideas a comonomer were polymerized in a twin-screw kneader having paddleswhich engage each other using boron trifluoride etherate as a catalystand methylal as a chain transfer agent. After the end of polymerization,the catalyst was deactivated with a benzene solvent containing a smallamount of triphenyl phosphine and milled to give a polyoxymethylenecopolymer. The polyoxymethylene copolymer had an intrinsic viscosity inp-chloroform (containing α-pinene) at 60° C. of 1.43 dl/g.

Example 1

0.3 part by weight of triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of CibaGeigy Co., Ltd., steric hindrance phenol under the trade name of Irganox245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part byweight of magnesium hydroxide and 0.2 part by weight of polyethyleneglycol having an average molecular weight of 10,000 were added to 100parts by weight of the polyoxymethylene copolymer produced in ReferenceExample 1 and pre-mixed with a Henschel mixer. Thereafter, the resultingmixture was melt kneaded at a cylinder temperature of 200° C. using atwin-screw extruder having a vent port, and pelletized to produce aresin composition. The color of the resin composition was adjusted byblending 0.3 part by weight of carbon black into the pellets and meltkneading with a twin-screw extruder again. The evaluation results areshown in Table 1.

Examples 2 and 3

Polyoxymethylene resin compositions were produced in the same manner asin Example 1 except that the average molecular weight of polyethyleneglycol was changed to 20,000 and 25,000, and evaluated. The evaluationresults are shown in Table 1.

Comparative Example 1

A polyoxymethylene resin composition was produced in the same manner asin Example 1 except that the average molecular weight of polyethyleneglycol was changed to 6,000 and evaluated. The evaluation results areshown in Table 1.

Comparative Examples 2 to 4

Polyoxymethylene resin compositions were produced in the same manner asin Example 1 except that polyethylene glycol was removed and the amountof melamine was changed to 0.0 to 0.1 part by weight, and evaluated. Theevaluation results are shown in Table 1.

Examples 4 to 7

Polyoxymethylene resin compositions were produced in the same manner asin Example 1 except that the amount of polyethylene glycol (averagemolecular weight of 20,000) was changed to 0.05 to 0.4 part by weight,and evaluated. The evaluation results are shown in Table 1.

Example 8 and Comparative Example 5

After 10 parts by weight of talc was mixed with the pellets of thepolyoxymethylene resin composition produced in Example 1, the resultingmixture was melt kneaded with a twin-screw extruder at a cylindertemperature of 200° C. and pelletized to produce a resin composition.The color of the resin composition was adjusted by blending 0.3 part byweight of carbon black with the pellets and melt kneading with atwin-screw extruder again. The evaluation results are shown in Table 1.For comparison, the evaluation results of a resin composition which didnot contain polyethylene glycol are shown in Table 1.

Example 9 and Comparative Example 6

0.3 part by weight of triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of CibaGeigy Co., Ltd., steric hindrance phenol under the trade name of Irganox245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part byweight of magnesium hydroxide and 0.2 part by weight of polyethyleneglycol having an average molecular weight of 20,000 were added to 100parts by weight of the polyoxymethylene copolymer produced in ReferenceExample 2 and pre-mixed with a Henschel mixer. Thereafter, the resultingmixture was melt kneaded at a cylinder temperature of 200° C. using atwin-screw extruder having a vent port, and pelletized to produce aresin composition. The color of the resin composition was adjusted byblending 0.3 part by weight of carbon black into the pellets and meltkneading with a twin-screw extruder again. The evaluation results areshown in Table 1. The evaluation results of a resin composition obtainedby eliminating polyethylene glycol are shown in Table 1 for comparison.

TABLE 1 Stabilizer Polyethylene Amine-substituted Steric hindranceInorganic Type of Glycol triazine compound Phenol Metal compound filledmaterial comonomer (part by weight) (part by weight) (part by weight)(part by weight) (part by weight) Ex. 1 1,3-dioxolan A-1 (0.20) B-1(0.10) C-1 (0.30) D-1 (0.05) Ex. 2 1,3-dioxolan A-2 (0.20) B-1 (0.10)C-1 (0.30) D-1 (0.05) Ex. 3 1,3-dioxolan A-3 (0.20) B-1 (0.10) C-1(0.30) D-1 (0.05) Ex. 4 1,3-dioxolan A-2 (0.05) B-1 (0.10) C-1 (0.30)D-1 (0.05) Ex. 5 1.3-dioxolan A-2 (0.10) B-1 (0.10) C-1 (0.30) D-1(0.05) Ex. 6 1,3-dioxolan A-2 (0.15) B-1 (0.10) C-1 (0.30) D-1 (0.05)Ex. 7 1,3-dioxolan A-2 (0.40) B-1 (0.10) C-1 (0.30) D-1 (0.05) Ex. 81,3-dioxolan A-2 (0.20) B-1 (0.10) C-1 (0.30) D-1 (0.05) E-1 (10.00) Ex.9 ethylene oxide A-2 (6.20) B-1 (0.10) C-1 (0.30) D-1 (0.05) Comp. Ex. 11,3-dioxolan A-4 (0.20) B-1 (0.10) C-1 (0.30) D-1 (0.05) Comp. Ex. 21,3-dioxolan B-1 (0.10) C-1 (0.30) D-1 (0.05) Comp. Ex. 3 1,3-dioxolanB-1 (0.05) C-1 (0.30) D-1 (0.05) Comp. Ex. 4 1,3-dioxolan C-1 (0.30) D-1(0.05) Comp. Ex. 5 1,3-dioxolan B-1 (0.10) C-1 (0.30) D-1 (0.05) E-1(10.00) Comp. Ex. 6 ethylene oxide B-1 (0.10) C-1 (0.30) D-1 (0.05)Shrink anisotropy (100 mm × 100 mm × 4 mm) Shrinkage factor Shrinkagefactor Thermal stability (minutes) (flow direction) (perpendiculardirection) Anisotropy Non-colored Black-colored Ex. 1 2.29% 2.20% 0.09%70 35 Ex. 2 2.26% 2.18% 0.08% 70 40 Ex. 3 2.26% 2.17% 0.09% 70 35 Ex. 42.33% 2.22% 0.11% 70 30 Ex. 5 2.29% 2.20% 0.09% 70 35 Ex. 6 2.27% 2.19%0.08% 70 35 Ex. 7 2.24% 2.18% 0.06% 70 40 Ex. 8 2.06% 2.00% 0.06% 50 20Ex. 9 2.27% 2.18% 0.09% 50 25 Comp. Ex. 1 2.32% 2.18% 0.14% 60 20 Comp.Ex. 2 2.29% 2.14% 0.15% 60 20 Comp. Ex. 3 2.30% 2.17% 0.13% 30 10 Comp.Ex. 4 2.34% 2.32% 0.02%  5 Unmeasurable Comp. Ex. 5 2.10% 1.99% 0.11% 4010 Comp. Ex. 6 2.29% 2.13% 0.16% 40 10 Ex.: Example Comp. Ex.:Comparative Example A-1: Polyethylene glycol (average molecular weightof 10,000) A-2: Polyethylene glycol (average molecular weight of 20,000)A-3: Polyethylene glycol (average molecular weight of 25,000) A-4:Polyethylene glycol (average molecular weight of 6,000) B-1: MelamineC-1: Irganox 245 D-1: Magnesium hydroxide E-1: Talc

Example 10

0.3 part by weight of triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of CibaGeigy Co., Ltd., steric hindrance phenol under the trade name of Irganox245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part byweight of magnesium hydroxide and 0.2 part by weight of polyethylene waxF-1 [of an acid value modified type, molecular weight of 4,000, acidvalue of 1.0 mg-KOH/g] were added to 100 parts by weight of thepolyoxymethylene copolymer produced in Reference Example 1 and pre-mixedwith a Henschel mixer. Thereafter, the resulting mixture was meltkneaded at a cylinder temperature of 200° C. using a twin-screw extruderhaving a vent port, and pelletized to produce a resin composition. Thecolor of the resin composition was adjusted by blending 0.3 part byweight of carbon black into the pellets and melt kneading with atwin-screw extruder again. The evaluation results are shown in Table 2.

Example 11

A polyoxymethylene resin composition was produced in the same manner asin Example 10 except that polyethylene wax F-2 [of an acid valuemodified type, molecular weight of 3,200, acid value of 12 mg-KOH/g] wasused in place of polyethylene wax F-1. The evaluation results are shownin Table 2.

Example 12

A polyoxymethylene resin composition was produced in the same manner asin Example 10 except that polyethylene wax F-3 [of an acid valuemodified type, molecular weight of 2,700, acid value of 30 mg-KOH/g] wasused in place of polyethylene wax F-1. The evaluation results are shownin Table 2.

Example 13

A polyoxymethylene resin composition was produced in the same manner asin Example 10 except that polypropylene wax F-4 [of an acid valuemodified type, molecular weight of 8,000, acid value of 2.0 mg-KOH/g]was used in place of polyethylene wax F-1. The evaluation results areshown in Table 2.

Examples 14 to 16

Polyoxymethylene resin compositions were produced in the same manner asin Example 10 except that the amount of polyethylene wax F-1 [of an acidvalue modified type, molecular weight of 4,000, acid value of 1.0mg-KOH/g] was changed to 0.05 to 0.4 part by weight. The evaluationresults are shown in Table 2.

Comparative Example 7

A polyoxymethylene resin composition was produced in the same manner asin Example 10 except that polyethylene wax F-5 [of a generalhigh-density type, molecular weight of 8,000, acid value of 0 mg-KOH/g]was used in place of polyethylene wax F-1. The evaluation results areshown in Table 2.

Comparative Example 8

A polyoxymethylene resin composition was produced in the same manner asin Example 10 except that high molecular weight polyethylene wax F-6[molecular weight of 100,000] was used in place of polyethylene wax F-1.The evaluation results are shown in Table 2.

Example 17 and Comparative Example 9

After 10 parts by weight of talc was mixed with the pellets of thepolyoxymethylene resin composition produced in Example 10, the resultingmixture was melt kneaded with a twin-screw extruder at a cylindertemperature of 200° C. and pelletized to produce a resin composition.The color of the resin composition was adjusted by blending 0.3 part byweight of carbon black with the pellets and melt kneading with atwin-screw extruder again. The evaluation results are shown in Table 2.For comparison, the evaluation results of a resin composition which didnot contain polyethylene wax are shown in Table 2.

Example 18 and Comparative Example 10

0.3 part by weight of triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of CibaGeigy Co., Ltd., steric hindrance phenol under the trade name of Irganox245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part byweight of magnesium hydroxide and 0.2 part by weight of polyethylene waxF-1 [of an acid value modified type, molecular weight of 4,000, acidvalue of 1.0 mg-KOH/g] were added to 100 parts by weight of thepolyoxymethylene copolymer produced in Reference Example 2 and pre-mixedwith a Henschel mixer. Thereafter, the resulting mixture was meltkneaded at a cylinder temperature of 200° C. using a twin-screw extruderhaving a vent port, and pelletized to produce a resin composition. Thecolor of the resin composition was adjusted by blending 0.3 part byweight of carbon black into the pellets and melt kneading with atwin-screw extruder again. The evaluation results are shown in Table 2.The evaluation results of a resin composition obtained by eliminatingpolyethylene wax are shown in Table 2 as comparison.

TABLE 2 Stabilizer Polyolefin Amine-substituted Steric hindranceInorganic Type of Wax triazine compound Phenol Metal compound filledmaterial comonomer (part by weight) (part by weight) (part by weight)(part by weight) (part by weight) Ex. 10 1,3-dioxolan F-1 (0.20) B-1(0.10) C-1 (0.30) D-1 (0.05) Ex. 11 1,3-dioxolan F-2 (0.20) B-1 (0.10)C-1 (0.30) D-1 (0.05) Ex. 12 1,3-dioxolan F-3 (0.20) B-1 (0.10) C-1(0.30) D-1 (0.05) Ex. 13 1,3-dioxolan F-4 (0.20) B-1 (0.10) C-1 (0.30)D-1 (0.05) Ex. 14 1,3-dioxolan F-1 (0.05) B-1 (0.10) C-1 (0.30) D-1(0.05) Ex. 15 1,3-dioxolan F-1 (0.10) B-1 (0.10) C-1 (0.30) D-1 (0.05)Ex. 16 1,3-dioxolan F-1 (0.40) B-1 (0.10) C-1 (0.30) D-1 (0.05) Ex. 171,3-dioxolan F-1 (0.20) B-1 (0.10) C-1 (0.30) D-1 (0.05) E-1 (10.00) Ex.18 ethylene oxide F-1 (0.20) B-1 (0.10) C-1 (0.30) D-1 (0.05) Comp. Ex.7 1.3-dioxolan F-5 (0.20) B-1 (0.10) C-1 (0.30) D-1 (0.05) Comp. Ex. 81,3-dioxolan F-6 (0.20) B-1 (0.10) C-1 (0.30) D-1 (0.05) Comp. Ex. 91,3-dioxolan B-1 (0.10) C-1 (0.30) D-1 (0.05) E-1 (10.00) Comp. Ex. 10ethylene oxide B-1 (0.10) C-1 (0.30) D-1 (0.05) Shrink anisotropy (100mm × 100 mm × 4 mm) Shrinkage factor Shrinkage factor Thermal stability(minutes) (flow direction) (perpendicular direction) AnisotropyNon-colored Black-colored Ex. 10 2.38% 2.30% 0.08% 70 35 Ex. 11 2.30%2.21% 0.09% 70 30 Ex. 12 2.26% 2.17% 0.09% 70 30 Ex. 13 2.27% 2.18%0.09% 70 30 Ex. 14 2.33% 2.22% 0.11% 65 25 Ex. 15 2.37% 2.28% 0.09% 7030 Ex. 16 2.39% 2.32% 0.07% 70 35 Ex. 17 2.18% 2.11% 0.07% 50 15 Ex. 182.27% 2.18% 0.09% 50 20 Comp. Ex. 7 2.32% 2.18% 0.15% 60 20 Comp. Ex. 82.32% 2.18% 0.16% 60 20 Comp. Ex. 9 2.22% 2.10% 0.12% 40  5 Comp. Ex. 102.29% 2.13% 0.16% 40 10 Ex.: Example Comp. Ex.: Comparative Example F-1:Polyethylene wax (of acid value modified type: average molecular weightof 4,000, acid value of 1.0 mg-KOH/g) F-2: Polyethylene wax (of acidvalue modified type: average molecular weight of 3,200, acid value of 12mg-KOH/g) F-3: Polyethylene wax (of acid value modified type: averagemolecular weight of 2,700, acid value of 30 mg-KOH/g) F-4: Polyethylenewax (of acid value modified type: average molecular weight of 8,000,acid value of 2.0 mg-KOH/g) F-5: Polyethylene wax (of acid valuemodified type: average molecular weight of 8,000, acid value of 0mg-KOH/g) F-6: High molecular weight polyethylene (average molecularweight of 100,000) B-1: Melamine C-1: Irganox 245 D-1: Magnesiumhydroxide E-1: Talc

Since the polyoxymethylene copolymer resin composition of the presentinvention has extremely low shrink anisotropy when it is left for a longtime after molding or in a high-temperature atmosphere and excellentthermal stability, it is suitable for use as a molding material for suchan application field that requires dimensional stability as precisionparts. The resin composition of the present invention is free from areduction in thermal stability, reductions in mechanical properties anddeterioration in the surface state of a molded product, all of which arethe problems of the prior art methods for improving shrink anisotropy byblending various resins and fillers. Therefore, it can be widely used insuch application fields such as automobiles, electric and electronicparts, construction materials and household goods in which apolyoxymethylene resin has been used.

What is claimed is:
 1. A polyoxymethylene resin composition comprising:(A) 100 parts by weight of a polyoxymethylene copolymer which contains0.4 to 10 mol % of oxyalkylene derived from 1,3-dioxolan on the mainchain of oxymethylene; (B) 0.01 to 7 parts by weight of anamine-substituted triazine compound; (C) 0.01 to 5 parts by weight of(C-1) polyethylene glycol having an average molecular weight of 10,000or more and/or (C-2) modified polyolefin wax having an acidic group withan acid value of 0.5 to 60 mg-KOH/g; (D) 0.01 to 5 parts by weight ofsteric hindrance phenol; and (E) 0.001 to 5 parts by weight of at leastone metal compound selected from the group consisting of hydroxides,inorganic acid salts, organic acid salts and alkoxides of alkali metalsand alkali earth metals.
 2. The polyoxymethylene resin composition ofclaim 1, wherein the polyoxymethylene copolymer (A) is produced bypolymerizing trioxan and 1,3-dioxolan as a copolymerizable component inthe presence of boron trifluoride or a coordination compound thereof asa catalyst.
 3. A polyoxymethylene resin composition comprising: (A) 100parts by weight of a polyoxymethylene copolymer which contains 0.4 to 10mol % of oxyalkylene units derived from 1,3-dioxolan in the main chainof oxymethylene; (B) 0.01 to 7 parts by weight of an amine-substitutedtriazine compound; (C) 0.01 to 5 parts by weight of polyethylene glycolhaving an average molecular weight of 10,000 or more; (D) 0.01 to 5parts by weight of steric hindrance phenol; and (E) 0.001 to 5 parts byweight of at least one metal compound selected from the group consistingof hydroxides, inorganic acid salts, organic acid salts and alkoxides ofalkali metals and alkali earth metals.
 4. A polyoxymethylene resincomposition comprising: (A) 100 parts by weight of a polyoxymethylenecopolymer which contains 0.4 to 10 mol % of oxyalkylene units derivedfrom 1,3-dioxolan in the main chain of oxymethylene; (B) 0.01 to 7 partsby weight of an amine-substituted triazine compound; (C) 0.01 to 5 partsby weight of modified polyolefin wax having an acidic group with an acidvalue of 0.5 to 60 mg-KOH/g; (D) 0.01 to 5 parts by weight of sterichindrance phenol; and (E) 0.001 to 5 parts by weight of at least onemetal compound selected from the group consisting of hydroxides,inorganic acid salts, organic acid salts and alkoxides of alkali metalsand alkali earth metals.
 5. A molded product formed from the resincomposition of claim 1, 3 or 4.